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UNIVERSITY  OF  ILLINOIS 

Agricultural  Experiment  Station 


BULLETIN  No.  285 


EXPERIMENTS  IN  HANDLING 
SWEET  CLOVER 

With  Reference  to  the  Accumulation  and 
Conservation  of  Nitrates  in  the  Soil 

BY  ALBERT  L.  WHITING  AND  THOMAS  E.  RICHMOND 


URBANA,  ILLINOIS,  JANUARY,  1927 


CONTENTS 

PAGE 

COMPARISON  OF  FALL-PLOWED  AND  SPRING-PLOWED  SWEET 
CLOVER  FOR  NITRATE  PRODUCTION . . . . 288 

NITRIFICATION  OF  SWEET  CLOVER  SPRING-PLOWED   AT   DIF- 
FERENT DATES 292 

Observations  on  the  Hartsburg  Field 292 

Observations  on  the  Toledo  Field 295 

Growth  Measurements  and  Nitrogen  Content  of  Sweet-Clover  Tops 297 

NITRIFICATION  OF  SUMMER-PLOWED  SWEET  CLOVER 299 

WINTER  LOSS  OF  NITRATE : 302 

SUMMARY  AND  CONCLUSIONS..  .  305 


EXPERIMENTS  IN  HANDLING 
SWEET  CLOVER 

With  Reference  to  the  Accumulation  and  Conservation 
of  Nitrates  in  the  Soil 

BY  ALBERT  L.  WHITING  AND  THOMAS  E.  RICHMOND* 

The  rapidly  increasing  use  of  sweet  clover  as  a  green  manure  testi- 
fies to  its  high  value  for  this  purpose.  Whereas  only  a  few  years  ago 
sweet  clover  was  despised  as  a  useless  weed,  so  rapidly  has  it  risen  in 
the  farmer's  estimation  that  in  the  year  1926  it  is  reported  that  744,000 
acres  in  Illinois  alone  were  given  over  to  the  cultivation  of  this  crop. 

Some  of  the  reasons  for  the  extraordinary  manurial  value  of  sweet 
clover  are  discussed  in  Bulletin  No.  233  of  this  Station  ("Sweet  Clover 
for  Nitrate  Production,"  1921).  A  principal  reason  is  to  be  attributed 
to  the  fact  that  sweet  clover  produces  such  a  large  amount  of  very 
readily  decomposable  organic  matter  which  upon  plowing  under  decays 
promptly  and  makes  available  in  the  soil  unusually  large  quantities  of 
nitrogen  for  the  nourishment  of  the  corn  or  other  crop  that  follows. 
The  above  mentioned  bulletin  presents  experimental  data  substantiat- 
ing this  fact,  and  showing  the  actual  amounts  of  nitrate  present  in  the 
soil  at  different  periods  thruout  the  season.  The  relative  amounts  of 
nitrate  produced  under  different  systems  of  soil  treatment  are  com- 
pared, thus  furnishing  information  on  the  economic  possibilities  of 
sweet  clover. 

There  are  various  questions  connected  with  the  culture  of  sweet 
clover,  however,  that  have  remained  unanswered — questions  pertaining 
to  the  proper  manner  of  handling  the  crop  in  order  to  realize  its  max- 
imum fertilizing  value.  Among  such  questions  is  that  of  the  proper 
time  of  plowing  down  sweet  clover  for  the  most  advantageous  produc- 
tion of  nitrate  in  relation  to  the  nourishment  of  the  succeeding  crop. 
For  instance,  would  fall  plowing  be  better  than  spring  plowing  from 
the  standpoint  of  nitrogen  supply  and  conservation?  Or,  if  plowed  in 
the  spring  is  earlier  plowing  better  than  later  plowing  in  this  regard? 

The  present  bulletin  reports  the  results  of  a  study  designed  to 
throw  some  light  on  these  questions.  The  relative  merits  of  fall  and 
spring  plowing  are  considered,  as  are  also  spring  plowing  at  different 
dates,  and  summer  plowing  of  the  second  year's  crop.  Consideration  is 
also  given  to  the  losses  of  nitrate  from  the  soil  and  to  means  of  pre- 
venting these  losses  thru  proper  farm  practices,  special  attention  being 
directed  to  the  role  that  sweet  clover  plays  in  conserving  soil  nitrates 
during  the  fall,  winter,  and  spring. 

IALBERT  L.  WHITING,  formerly  Chief  in  Soil  Biology;  THOMAS  E.  RICHMOND, 
formerly  Associate  in  Soil  Biology. 

287 


288  BULLETIN  No.  285  [January, 

In  carrying  out  these  studies,  observations  were  made  on  a  num- 
ber of  outlying  soil  experiment  fields  located  in  different  sections  of 
Illinois.  These  observations  involved  the  taking  of  soil  samples  at 
various  intervals  of  time,  as  indicated  in  the  respective  tables  of  re- 
sults which  follow,  and  the  determination  of  the  nitrate  content.1 


COMPARISON  OF  FALL-PLOWED  AND  SPRING-PLOWED 
SWEET  CLOVER  FOR  NITRATE  PRODUCTION 

In  order  to  compare  the  effect  of  fall  plowing  with  spring  plowing 
of  sweet  clover  with  respect  to  its  value  for  nitrate  production  for 
corn,  certain  plots  of  Series  300  of  the  Joliet  field  were  selected. 

These  plots  had  produced  under  their  respective  full  soil  treat- 
ments the  following  succession  of  crops:  corn,  oats,  soybeans,  wheat. 
In  the  spring  of  1921  sweet  clover  was  seeded  in  the  wheat  as  a  catch 
crop  to  be  plowed  under  for  the  benefit  of  the  corn  to  follow.  The 
plots  selected  for  the  study  were  the  following: 

Plot  305— No  treatment 

Plot  307 — Limestone  and  sweet  clover 

Plot  308 — Limestone,  phosphorus,  residues,  and  sweet  clover 

Plot  309 — Limestone,  phosphorus,  potassium,  residues,  and  sweet  clover 

For  making  the  desired  comparisons  these  plots  were  divided  into 
east  and  west  halves.  The  east  halves  were  all  fall-plowed  in  Novem- 
ber and  the  west  halves  were  spring-plowed  in  the  following  May. 

The  condition  of  the  sweet  clover  was  excellent  on  Plot  309,  good 
on  Plot  308,  and  fair  on  Plot  307.  An  excellent  job  of  fall  plowing 
was  done,  the  sweet-clover  roots  being  cut  off  completely  at  a  depth  of 
about  7  inches  and  the  tops  turned  under  properly.  Samples  of  soil 
were  taken  November  25  on  both  halves  of  the  plots. 

In  the  early  spring  the  fall-plowed  sweet  clover  began  to  grow, 
sending  out  many  shoots  from  the  crowns  of  the  old  roots  and  devel- 
oping new  feeding  roots  from  the  old  root  stocks.  A  severe  winter  had 
not  killed  it,  and  it  grew  so  rapidly  that  it  made  a  fair  stand.  The 
ground  was  double-disked,  but  still  the  sweet  clover  continued  to  grow 
until  finally  in  order  to  subdue  it  the  land  was  replowed.  This  was 
done  about  May  7,  at  the  same  time  that  the  west  half-plots  were 
plowed.  This  experience  in  the  fall  plowing  of  sweet  clover  was  cer- 
tainly not  encouraging  as  a  practice  to  be  adopted. 

On  May  5  soil  samples  were  taken  before  the  spring  plowing.  The 
unplowed  clover  was  14  inches  high  on  Plot  309  West,  12  inches  on 
Plot  308  West,  and  only  about  8  inches  in  height  and  irregular  in  stand 


'For  a  description  of  the  technic  employed,  including  the  methods  of  col- 
lecting soil  samples  and  making  analysis  for  nitrate  nitrogen,  the  reader  is 
referred  to  an  article  entitled  "The  Determination  of  Nitrates  in  Soil,"  Jour. 
Indus,  and  Engin.  Chem.  12,  982. 


HANDLING  SWEET  CLOVER  FOR  NITRATE  PRODUCTION  289 

on  Plot  307  West.    Spring  rains  kept  the  nitrates  low  up  to  May  5  and 

after. 

On  July  6  the  following  measurements  of  the  height  of  the  corn 

were  taken  which  bring  out  well  the  comparative  differences  in  growth : 

Plot  305  East 17  inches 

Plot  305  West 20  inches 

Plot  307  East 18  inches 

Plot  307  West 32  inches 

Plot  308  East 24  inches 

Plot  308  West 36  inches 

Plot  309  East 30  inches 

Plot  309  West 40  inches 

The  differences  in  vegetative  growth  between  the  east  and  west 
halves  were  evident  a  considerable  distance  from  the  field  until  the 
corn  became  mature.  Corn  on  the  adjacent  Series  200  was  much  slower 
in  its  growth  than  the  corn  on  the  west  side  of  Series  300. 

The  results  of  the  nitrate  determinations  on  the  east  and  west 
halves  of  the  respective  plots  are  given  in  Table  1  in  terms  of  pounds 
per  acre  of  nitrate  nitrogen  in  the  surface  soil. 

The  nitrate  content  in  the  fall  (on  November  25)  was  low  in  the 
surface  soil  and  only  slight  differences  were  found  between  the  east  and 
west  halves.  These  results  would  indicate  that  the  nitrate  content  of 
the  surface  soil  had  been  reduced  to  a  nearly  uniform  amount  as  an 
effect  of  the  11  inches  of  rain  from  September  to  December.  While 
the  check  plots  showed  16.2  and  15.2  pounds  of  nitrate  nitrogen  per 
acre  in  the  surface  soil,  it  is  to  be  noted  that  the  treated  plots  con- 
tained about  the  same  amount.  But  in  addition  there  probably  was 
present  on  the  treated  plots  at  least  100  pounds  of  nitrogen  per  acre  in 
the  sweet-clover  crop,  whether  plowed  or  unplowed.  This  is  a  most  sig- 
nificant fact  that  should  not  be  overlooked  in  considering  the  sweet- 
clover  plots,  for  the  nitrogen  contained  in  the  sweet-clover  plants  is 
largely  saved  for  another  year. 

In  the  spring,  after  12.6  inches  of  rain,  and  with  the  growth  of  the 
crop  on  the  treated  plots,  the  nitrate  was  reduced  on  the  west  halves  of 
Plots  308  and  309  to  very  low  figures,  apparently  due  largely  to  the 
clover  growth.  On  Plot  307,  where  the  poorest  sweet  clover  was  grow- 
ing, the  figures  indicate  a  small  increase  in  nitrate.  The  check  plots 
show  very  little  increase.  With  the  advance  of  the  season,  attended  by 
rising  temperatures,  the  usual  increase  in  nitrate  content  of  the  soil  was 
manifested.  The  treated  plots  show  a  decidedly  higher  nitrate  content 
than  the  untreated  thruout  the  season.  The  phosphorus  and  potassium 
plots  (Plots  308  and  309) ,  in  spite  of  the  fact  that  they  were  support- 
ing the  heaviest  growth  of  corn,  show  the  highest  nitrate  content  of  all 
plots  during  the  important  feeding  period  of  the  corn  crop. 

Unfortunately  there  is  some  variation  in  soil  type  on  the  Joliet 
field  which  has  been  revealed  thru  a  careful  resurvey  made  subsequent 


290 


BULLETIN  No.  285 


[January, 


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1927}  HANDLING  SWEET  CLOVER  FOR  NITRATE  PRODUCTION  291 

to  these  experiments.  Probably  the  most  serious  disturbance  from 
this  cause  would  be  found  on  Plot  309,  where  the  east  half  is  almost 
entirely  occupied  by  the  soil  type  now  mapped  as  Black  Clay  Loam, 
poorly  drained  phase,  while  the  west  half  is  Brown  Silt  Loam  On 
Plastic  Calcareous  Drift.  It  is  not  known  to  what  extent  the  effect 
of  these  natural  soil  variations  may  obscure  these  experimental  re- 
sults; therefore,  a  very  critical  comparison  of  the  data  for  Plot  309  is 
precluded.  It  is  possible  that  the  extreme  variation  exhibited  in  the 
August  18  samples  for  Plot  309  may  be  due  to  this  difference  in  soil. 
For  the  same  reason  it  seems  futile  to  compare  the  subsoil  data  for 
the  east  and  west  halves.  The  figures,  however,  are  not  devoid  of 
interest,  for  both  half-plots  show  a  liberal  excess  of  available  nitrogen 
present  in  the  subsoil  as  well  as  in  the  surface  soil  in  the  period  of 
greatest  demand  by  the  growing  corn  crop. 

The  physical  condition  of  the  soil  on  the  east  side  of  the  plots 
was  very  bad,  while  it  was  ideal  on  the  west  side.  The  spring  disking 
and  replowing  of  the  east  side  resulted  in  a  cloddy  condition  that  lasted 
all  thru  the  season,  and  the  large  differences  in  the  condition  of  the 
corn  in  its  early  growth  on  the  two  halves  may  have  been  largely  due 
to  the  poor  physical  condition  of  the  east  half.  It  is  evident  that  the 
fall  growth  of  sweet  clover,  combined  with  its  spring  growth,  in  spite 
of  two  double-diskings  on  the  east  halves,  furnished  nitrates  enough 
for  about  twice  the  crop  grown.  The  corn  yields  did  not  vary  on  the 
east  and  west  halves  by  significant  differences,  the  yield  being  only  1.8 
bushels  greater  on  the  east  side  of  Plot  308,  and  1.4  bushels  greater  on 
the  west  side  of  Plot  309. 

The  suggestion  arises  that  perhaps  lack  of  sufficient  phosphorus, 
potassium,  or  other  mineral  elements  of  plant  food  may  be  limiting 
crop  growth  on  this  field.  Such  results  as  these  nitrate  data  show  are 
encouraging  from  the  standpoint  of  the  nitrogen  problem,  indicating, 
as  do  also  the  results  reported  in  Bulletins  225  and  233,  that  with 
sweet  clover  used  as  a  green  manure,  nitrification  runs  well  ahead  of 
crop  requirements  where  proper  soil  treatment  is  applied. 

The  nitrate  results  for  August  18  are  very  high  for  this  time  of 
the  season.  The  possibility  is  suggested  that  this  large  amount  of 
nitrate  in  the  surface  soil  may  be  due  to  a  rise  of  nitrate  from  the 
subsoil.  The  surface  soil  contained  only  8.5  to  11.9  percent  of  moisture 
on  August  18.  Such  a  low  moisture  content  will  not  support  nitrifica- 
tion in  this  soil.  Other  cases  of  nitrate  concentration  in  the  surface 
soil  will  be  presented  in  the  data  from  other  fields.  Such  an  upward 
movement  of  nitrates  is  an  important  consideration,  especially  if  it 
occurs  at  this  time  of  year.  The  possibility  of  conserving  the  excess 
nitrogen  for  another  year  by  planting  a  fall  cover  crop  might  well  be 
considered  in  this  connection. 


292  BULLETIN  No.  285  [January, 

With  fall  plowing  on  this  field  there  was  furnished  more  nitrate 
than  was  needed  for  one  large  corn  crop.  In  view  of  this  fact  it  would 
be  desirable  to  find  a  satisfactory  method  of  fall  plowing,  by  which 
the  sweet  clover  roots  would  be  thoroly  killed,  since  in  some  rotations 
the  amount  of  nitrates  developed  under  this  practice  would  be  ample 
to  meet  all  crop  requirements.  The  present  study  indicates,  however, 
that  spring  plowing  of  green  sweet  clover  is  better  than  fall  plowing, 
considered  from  all  standpoints,  including  nitrate  production,  physical 
condition  of  soil,  and  amount  of  mechanical  labor  involved. 

NITRIFICATION  OF  SWEET  CLOVER  SPRING-PLOWED 
AT  DIFFERENT  DATES 

The  effect  of  varying  the  date  of  plowing  in  the  spring,  on  the 
value  of  green  sweet  clover  as  a  nitrate  producer,  was  studied  in  1921 
on  certain  plots  of  the  Hartsburg  experiment  field,  in  Logan  county, 
and  of  the  Toledo  field,  in  Cumberland  county. 

OBSERVATIONS  ON  THE  HARTSBURG  FIELD 

The  soil  of  the  Hartsburg  field  is  of  a  heavy  type,  being  classified 
as  Black  Clay  Loam.  Sweet  clover  usually  grows  fairly  well  on  the 
unlimed  plot  on  this  field. 

The  east  side  of  Series  300  was  plowed  on  May  4  and  the  west 
side  May  13.  Samples  of  the  sweet  clover  were  taken  for  analysis  to 
determine  the  rate  of  nitrogen  gain  per  acre.  Unfortunately  for  this 
study  some  of  the  sweet  clover  was  frozen  three,  and  some  four,  times. 
The  green  weights  and  water- free  weights  of  the  crop  at  time  of  plow- 
ing were  determined  by  cutting  areas  one  yard  square,  obtaining  the 
green  weight  in  the  field  and  the  water-free  weight  after  oven-drying. 
These  results,  together  with  the  nitrate  determinations  in  the  surface 
soil,  are  arranged  in  Table  2. 

The  first  soil  samples  were  taken  on  May  3.  The  differences  be- 
tween the  east  and  west  sides  were  not  very  pronounced  at  this 
sampling.  The  sweet  clover  stand  was  irregular,  and  this  may  have 
contributed  to  some  of  the  apparent  variations.  The  treated  plots  on 
which  sweet  clover  had  grown  very  well  in  the  previous  fall,  and  fairly 
well  in  the  spring,  contained  more  nitrates  than  the  checks,  where  a 
poor  growth  of  sweet  clover  was  present. 

On  the  treated  plots  the  nitrates  increased,  as  anticipated — more 
rapidly  on  the  east  halves  after  plowing  than  on  the  unplowed  west 
halves,  up  to  May  17.  Both  east  and  west  halves,  however,  increased 
in  nitrates  from  the  previous  date  of  sampling;  even  the  west  sides 
about  doubled  in  nitrate  content.  The  cold  weather  seems  to  have  had 
more  effect  in  checking  the  growth  of  the  sweet  clover,  than  it  had  in 
reducing  the  activity  of  the  nitrifying  bacteria.  Following  the  May  17 


1927} 


HANDLING  SWEET  CLOVER  FOR  NITRATE  PRODUCTION 


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294 


BULLETIN  No.  285 


sampling,  heavy  rains  reduced  the  nitrate  materially  on  all  plots  except 
Plot  309,  where  the  best  growth  of  sweet  clover  had  been  produced. 
The  large  amount  of  green  organic  matter  may  have  served  to  reduce 
the  apparent  loss  of  nitrate.  On  June  22  the  west  side,  representing 
the  later  plowing,  contained  much  the  larger  amounts  of  nitrate,  ex- 
cept on  Plot  308,  where  both  sides  were  very  high,  and  on  Plot  310, 
where  both  sides  were  low.  On  the  treated  plots  nitrates  continued  to 
accumulate  on  the  east  sides  altho  to  a  smaller  extent  than  on  the 
west  sides.  From  July  1  to  July  15  conditions  were  highly  favorable 
for  the  rise  of  nitrate  from  the  substrata  thru  evaporation,  altho  some 
nitrate  was  probably  produced,  inasmuch  as  the  moisture  did  not  fall 
to  a  prohibitive  point  on  this  field  as  it  did  on  the  Joliet  field. 


TABLE  3. — NITRATE  NITROGEN  IN  TREATED  AND  UNTREATED  PLOTS 

ON  HARTSBURG  FIELD,  1921 

(Pounds  per  acre  in  2  million  pounds  of  surface  soil  (about  0  to  63  inches) 
water-free  basis.) 


Average  of  — 

May 
3 

May 
17 

June 
8 

June 
22 

July 
1 

July 
15 

July 
29 

Aug. 
24 

Sept. 
30 

Treated  plots  

30.90 

72.70 

43.30 

74.80 

78.99 

91.88 

80.45 

43,57 

31.50 

17  65 

58  58 

26  35 

38  65 

41  60 

63  45 

49  40 

23  78 

25  08 

Increase  for  treatment  .  . 

13.25 

14.12 

16.95 

36.15 

37.39 

28.43 

31.05 

19.79 

6.42 

The  large  quantities  of  nitrate  present  are  indicative  of  the  suc- 
cess attained  in  its  production  by  proper  soil  treatment,  including  the 
use  of  green  sweet  clover.  In  view  of  the  fact  that  a  corn  crop  of  50 
to  63  bushels  per  acre  was  being  produced  on  this  land,  it  is  evident 
that  lack  of  nitrate  was  not  a  limiting  factor  in  production.  The  pres- 
ence of  such  large  amounts  of  nitrate  nitrogen  during  the  critical  feed- 
ing period  of  the  growing  crop  demonstrates  that  on  this  kind  of  soil 
the  nitrogen  problem  in  production  is  largely  solved  thru  the  use  of 
sweet  clover,  at  least  until  the  day  when,  thru  further  soil  and  crop 
improvement,  much  larger  crops  than  are  now  grown  shall  be  possible. 

During  August  and  September  heavy  rainfall  markedly  reduced 
the  nitrate  content  of  the  surface  soil,  falling  from  as  high  as  108 
pounds  to  as  low  as  29  pounds  per  acre  (Plot  308  East) .  If  the  nitrate 
represented  in  this  difference  of  79  pounds  was  permanently  lost, 
it  means  that  a  serious  depletion  of  the  most  expensive  plant- food 
element  takes  place,  and  the  conservation  of  this  fugitive  element, 
nitrogen,  by  crop  growth,  or  by  bacterial  action,  or  by  both,  is  of  deep 
concern  as  the  next  important  step  for  consideration  in  the  solution 
of  the  nitrogen  problem. 

The  average  nitrate  content  of  the  treated  compared  with  un- 
treated plots  is  given  in  Table  3.  The  results  show  that  thruout  the 
season  the  nitrate  in  the  soil  of  the  treated  plots  exceeded  that  of  the 
untreated  plots.  The  comparisons  are  especially  interesting  because 


1927]  HANDLING  SWEET  CLOVER  FOR  NITRATE  PRODUCTION  295 

the  soil  here  is  not  acid,  which  means  that  the  increases  are  largely 
due  directly  to  the  sweet  clover.  It  is  to  be  noted  that  these  results 
are  obtained  on  a  type  of  soil  regarded  as  naturally  highly  productive. 
From  the  nitrogen  standpoint  there  was  no  important  advantage 
in  either  the  May  4  or  the  May  13  plowing  date,  since  nitrates  were 
produced  in  excess  for  the  critical  feeding  period  whether  the  crop  was 
plowed  under  at  the  earlier  or  the  later  date.  There  may  be  other 
considerations,  however,  in  allowing  the  crop  to  remain  as  long  as  pos- 
sible before  plowing.  The  organic  matter,  nitrogen,  phosphorus,  and 
other  elements  contained  in  the  sweet  clover  would  increase  in  amount. 
The  gain,  of  course,  would  be  largest  in  the  organic  matter,  the  amount 
of  air-dry  material  often  increasing  from  1%  to  2  tons  an  acre  in  early 
spring  to  as  much  as  5  tons  an  acre  in  late  summer. 

OBSERVATIONS  ON  THE  TOLEDO  FIELD 

The  Toledo  Experiment  Field,  in  Cumberland  county,  in  south- 
eastern Illinois,  is  located  on  the  soil  type  designated  as  Gray  Silt 
Loam  On  Tight  Clay.  This  type  of  soil,  without  treatment,  produces 
poor  yields  in  general,  and  without  lime  sweet  clover  will  not  grow 
at  all. 

The  south  half  of  Series  300  was  plowed  on  May  1,  and  the  north 
half  on  May  16.  Sweet-clover  samples  were  taken,  the  data  concern- 
ing which  are  given  in  Table  4.  The  dry  weight  of  the  sweet  clover 
tops  doubled  in  the  15-day  interval. 

On  May  14,  thirteen  days  after  plowing,  the  nitrate  content  of  the 
south  sides,  where  sweet  clover  had  been  plowed  under,  was  greater 
than  that  of  the  corresponding  north  sides,  where  it  was  allowed  to 
stand.  This  ascendency,  however,  disappeared  by  the  next  date  of 
sampling.  Ample  nitrate  was  produced  on  both  sides  of  the  plots.  A 
very  poor  crop  of  corn  was  present  on  the  check  plots,  which  accounts 
in  part  for  their  high  nitrate  content,  but  some  of  the  nitrate  accumu- 
lation on  the  checks  is  to  be  ascribed  perhaps  to  the  rest  period  which 
this  series  had  enjoyed  thru  previous  partial  crop  failures.  On  this 
field,  the  different  dates  of  plowing  did  not  show  sufficient  difference 
in  the  nitrate  content  to  affect  the  crop.  A  liberal  surplus  was  present 
at  all  times  during  the  critical  feeding  period  of  the  corn  crop. 

The  excessive  amount  of  nitrate  in  the  surface  soil  appears  to 
have  been  due  to  a  rise  from  the  lower  layers  rather  than  to  a  direct 
production  of  nitrate  in  the  surface,  because  the  soil  moisture  was  very 
low  during  July,  ranging  from  6.4  percent  on  Plot  309  to  11.6  percent 
on  the  check  Plot  310.  The  rainfall  for  July  was  1.92  inches,  while  it 
was  5.94  inches  for  August  and  8.57  inches  for  September.  Here  again, 
as  suggested  above,  the  accumulation  under  excessively  dry  soil  condi- 
tions may  be  explained  by  a  rise  of  nitrates  from  lower  levels.  The 
great  losses  of  nitrate  from  the  surface  soil  due  to  rain  are  again  ap- 


296 


BULLETIN  No.  285 


[January, 


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19S7] 


HANDLING  SWEET  CLOVER  FOR  NITRATE  PRODUCTION 


297 


parent.  Plots  307,  308,  and  309,  averaged  104.8  pounds  of  nitrate 
nitrogen  on  July  14,  but  only  38.7  pounds  on  August  25,  and  37.7 
pounds  on  September  29.  Thus  an  apparent  loss  of  about  67  pounds 
occurred  during  this  period.  On  this  type  of  soil,  with  its  impervious 
subsoil,  the  nitrate  may  not  leach  away  altogether,  but  it  may  be  lost 
thru  denitrification  when  conditions  of  moisture  and  temperature  are 
favorable  for  this  process.  On  properly  drained  land  a  material  loss 
from  leaching  would  be  expected. 

In  considering  the  results  on  the  Toledo  field,  as  well  as  those  of 
the  Hartsburg  field,  it  might  be  well  to  point  out  that  since  sweet 
clover  had  been  grown  several  times  as  a  green  manure,  a  generally 
higher  level  of  nitrate  accumulation  existed  on  both  of  these  fields  than 
is  usually  found  in  soils  not  so  treated,  and  this  condition  probably 
reduced  the  possibility  of  larger  differences  in  nitrate  content  resulting 
from  the  effect  of  prolonging  the  spring  growth  period. 

TABLE  5. — NITRATE  NITROGEN  IN  TREATED  AND  UNTREATED 

PLOTS  ON  TOLEDO  FIELD,  1921 

(Pounds  per  acre  in  2  million  pounds  of  surface  soil  (about  0  to  6f  inches) 
water-free  basis) 


Average  of  — 

April 
22 

May 
4 

May 
14 

June 
7 

June 
23 

June 
30 

July 
14 

July 

27 

Aug. 
25 

Sept. 
29 

Treated  plots  .  . 
Untreated  plots 

23.10 
14.15 

21.70 
16.20 

56.20 
34.40 

51.00 
19.60 

41.60 
19.00 

67.50 
31.00 

104.90 
65.90 

85.65 
44.40 

38.71 
23.95 

37.38 
24.21 

Increase  for 
treatment.  .  . 

8.95 

5.50 

21.80 

31.40 

22.60 

36.50 

39.00 

41.25 

14.76 

13.17 

In  Table  5  the  average  nitrate  nitrogen  contents  of  the  treated  and 
the  untreated  plots  on  the  Toledo  field  are  reported.  The  treated 
plots  growing  sweet  clover  show  much  the  larger  amounts  of  nitrate 
present,  in  spite  of  the  fact  that  these  plots  were  supplying  nitrate  for 
the  production  of  much  larger  crops  than  were  the  check  plots. 

It  is  evident  that  on  this  field,  as  on  the  others  studied,  the  nitro- 
gen requirement  has  been  much  more  successfully  met  than  have 
those  of  other  factors  concerned  in  production.  Lack  of  sufficient 
moisture,  deficiency  of  available  phosphorus  or  other  elements  of  plant 
food,  injury  due  to  hot  winds,  or  any  one  of  the  innumerable  factors 
of  production,  working  singly  or  in  combination,  must  be  regarded  as 
the  cause  of  limited  crop  growth  on  this  field,  rather  than  a  shortage 
of  the  essential  nitrogen. 


GROWTH  MEASUREMENTS  AND  NITROGEN  CONTENT  OF 
SWEET-CLOVER  TOPS 

Certain  observations  with  respect  to  the  spring  growth  of  the 
sweet  clover  on  the  Hartsburg  and  Toledo  fields  were  made,  including 
the  height,  green  weight,  dry  weight,  and  nitrogen  content  of  the  sweet- 


298 


BULLETIN  No.  285 


[January, 


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Hartsburg  

1927]  HANDLING  SWEET  CLOVER  FOR  NITRATE  PRODUCTION  299 

clover  tops.  The  data  for  these  measurements,  together  with  the  cal- 
culated nitrogen  per  acre  for  the  respective  plots,  are  recorded  in 
Table  6.  In  looking  over  these  results,  the  progressive  increase,  with 
advancing  season,  in  the  weight  of  nitrogen  per  acre  in  the  tops  is  ap- 
parent. On  the  Toledo  field,  at  the  middle  of  May,  there  was  con- 
tained in  the  tops,  from  67  to  138  pounds  of  nitrogen  per  acre.  On 
the  Hartsburg  field,  60  to  85  pounds  of  nitrogen  per  acre  had  accumu- 
lated in  the  tops  by  May  3. 

On  depleted  soils,  where  the  active  organic-matter  content  is  low, 
it  would  seem  especially  important  that  the  crop  be  plowed  under  late 
the  first  time  it  is  used,  since  on  such  soils  one  of  the  principal  objects 
is  to  build  up  the  supply  of  active  organic  matter  as  rapidly  as  pos- 
sible. 


NITRIFICATION  OF  SUMMER-PLOWED  SWEET  CLOVER 

Thru  the  cooperation  of  the  Bloomington  Canning  Company  and 
the  McLean  County  Farm  Bureau,  an  opportunity  was  given  to  study 
nitrate  accumulation  and  losses  where  second-year  sweet  clover,  at  a 
rather  advanced  stage,  was  plowed  under.  The  sweet  clover  was 
about  six  feet  high  when  plowing  began,  about  July  22.  The  tops  were 
still  green  and  seed  had  not  yet  formed.  The  soil  of  this  field  is  Brown 
Silt  Loam,  and  it  had  been  limed.  There  were  100  acres  of  the  section 
in  second-year  sweet  clover. 

The  question  arose  as  to  the  loss  of  nitrates  that  might  result  from 
plowing  while  the  crop  was  still  green,  and  with  about  ten  months  in- 
tervening before  sweet  corn  would  be  planted.  It  was  suggested  that 
oats  and  rye  be  seeded  on  adjacent  plots,  these,  together  with  a  fallow 
check  plot,  to  be  sampled  for  nitrate  in  order  to  determine  the 
comparative  value  of  these  two  crops  for  conserving  the  nitrates  that 
would  be  produced  from  the  green  sweet  clover.  Plots  of  about  one- 
fourth  acre  in  area  were  arranged  at  the  northeast  corner  of  the  field. 
Samples  were  taken  soon  after  plowing,  and  again  on  August  25  (about 
one  month  later)  before  the  oats  and  rye  were  seeded.  The  surface 
soil  contained  77.8  pounds  of  nitrate  nitrogen  per  acre  on  July  29,  and 
49.1  pounds  on  August  25.  There  were  10.44  inches  of  rain  in  August, 
which  accounts  for  the  pronounced  reduction  noted.  Beginning  Oc- 
tober 1,  a  series  of  samples  was  taken  in  three  strata  to  a  depth  of  40 
inches  on  all  plots,  and  as  deep  as  80  inches  on  the  rye  plot.  The  data 
are  presented  in  Table  7. 

It  would  appear  that  sweet  clover  increased  the  nitrate  content 
of  the  fallow  soil,  since  on  November  3,  103.3  pounds  per  acre  were 
found  in  the  surface  stratum,  107.2  in  the  middle  stratum,  and  92  in 


300  BULLETIN  No.  285  [January, 

the  lower  stratum,  or  a  total  of  302.5  pounds  in  the  40  inches.  This 
was  two  to  three  times  as  much  as  was  found  on  the  highest  producing 
experiment  fields  sampled  at  the  same  time  in  the  same  manner.  The 
much  lower  nitrate  content  of  the  oat  and  rye  plots  at  this  sampling 
is  made  evident  by  the  figures  for  the  upper  and  middle. strata.  The 
amount  of  nitrate  in  the  lower  strata  (20  to  40  inches)  is  about  the 
same  as  that  of  the  fallow,  which  fact  may  be  accounted  for  by  the 
heavy  rains  having  washed  the  soil  equally  on  all  plots  before  the  oats 
and  rye  had  grown  sufficiently  to  use  nitrate,  and  to  offer  protection 
against  its  descent.  The  oats  in  their  fall  growth  attained  a  height  of 
14  inches  and  covered  the  ground.  The  rye  made  a  thin  stand  at  first, 
but  stooled  out  to  make  a  fair  stand  about  4  inches  high  by  November 
3.  At  this  date  there  was  a  difference  in  nitrate  present  to  a  depth  of 
40  inches  of  88.8  pounds  between  the  fallow  and  the  rye  plot,  and  119.2 
pounds  between  the  fallow  and  the  oats  plot.. 

In  the  spring  the  oat  plot,  as  expected,  contained  the  highest  ni- 
trate content  in  the  surface  soil.  The  rye  grew  until  disked  in,  and  the 
nitrate  content  on  the  rye  plot  was  only  9.7  pounds.  The  fallow  plot 
suffered  heavily  from  leaching  and  contained  only  7.8  pounds  in  the 
surface  stratum.  In  July,  with  the  sweet  corn  growing  rapidly,  there 
was  a  liberal  excess  of  soil  nitrate  on  all  plots. 

The  oat  plot  was  the  most  efficient  in  the  utilization  of  the  nitrate. 
This  plot  on  May  4  contained  about  50  pounds  per  acre  more  nitrate 
than  the  fallow  plot.  On  July  5  it  was  the  lowest  of  the  three  plots, 
which  again,  as  in  the  previous  year,  would  mean  smaller  losses  be- 
cause of  the  greater  conversion  of  nitrate  nitrogen  into  organic 
nitrogen. 

The  thing  to  be  desired  with  respect  to  soil  nitrogen  is  that  it 
shall  be  held  in  the  fall  largely  in  an  insoluble  form,  and  that  it  shall 
gradually  become  available  the  following  season  in  amounts  just  suffi- 
cient to  keep  pace  with  the  demands  of  the  growing  crop.  When  an 
oats  cover  crop  was  included  in  the  management  of  the  soil  of  this 
Bloomington  field,  there  was  exhibited  in  a  high  degree  this  potential 
capacity  to  produce  ample  nitrate  as  needed. 

For  easier  comparison  of  the  data  showing  the  distribution  of  ni- 
trates in  the  soil  at  the  time  of  the  November  3  sampling,  the  figures 
given  in  Table  7  are  computed  in  pounds  of  nitrate  per  acre,  as  shown 
in  Table  8. 

The  distribution  of  the  nitrate  nitrogen  in  the  different  soil  layers 
as  represented  in  Table  8  shows  the  efficiency  of  these  cover  crops  in 
checking  the  downward  movement  of  the  nitrates.  The  principle  of  the 
conservation  of  nitrates  thru  the  presence  of  a  cover  crop  is  brought 
out  by  these  figures.  With  a  growing  crop  on  the  surface  soil,  a  con- 
version of  nitrate  nitrogen  to  organic  nitrogen  is  taking  place  at  the 
same  time  that  a  reduced  nitrification  occurs,  thus  building  up  the  sup- 


1927] 


HANDLING  SWEET  CLOVER  FOR  NITRATE  PRODUCTION 


301 


ply  of  organic  nitrogen  for  nitrification  at  a  later  period  to  meet  future 
needs.  Volunteer  oats  and  other  grains  function  in  a  limited  degree  in 
this  manner.  In  cases  where  land  is  not  to  be  used  for  growing  other 
crops,  sweet  clover  allowed  to  reach  maturity  will  greatly  increase  the 
organic-matter  content  of  the  soil,  and  provide  sufficient  nitrogen  for 

TABLE  7. — COMPARISON  OP  EFFECT  OF  FALLOW  AND  COVER-CROP  TREATMENTS 
ON  NITRATE  NITROGEN  IN  SOIL;  SWEET  CLOVER  PLOWED  UNDER  DURING 
LATTER  PART  OF  JULY,  1921:    BLOOMiNGTON,1  1921-22 
(Pounds  per  acre,  water-free  basis) 


Treatment 

Oct.  1 
1921 

Nov.  3 
1921 

May  4 
1922 

July  5 
1922 

Fallow 

78  5 

103.3 

7.8 

40.9 

Middle  sampling  stratum,  7  to  20  inches  

83  7 

107.2 

29.2 

60.5 

Lower  sampling  stratum,  20  to  40  inches  

60.2 

92.5 

56.3 

77.9 

Total  for  3  strata,  0  to  40  inches  

222  4 

303.0 

93.3 

179.3 

Rye  Cover  Crop 

64  6 

34.5 

9.7 

51.5 

Middle  sampling  stratum,  7  to  20  inches  

107.2 

94.0 

34.7 

61.3 

Lower  sampling  stratum,  20  to  40  inches  

99.0 

85.7 

62.4 

82.2 

Total  for  3  strata,  0  to  40  inches  

270  8 

214.2 

106.8 

196.0 

Substratum,  40-80  inches  

182.4 

182.2 

127.8 

160.3 

Oats  Cover  Crop 
Upper  sampling  stratum,  0  to  7  inches  

56.2 

29.5 

27.9 

50  3 

Middle  sampling  stratum,  7  to  20  inches  

84.6 

63.1 

48.9 

46.6 

Lower  sampling  stratum,  20  to  40  inches  

85.2 

91.2 

69.9 

50.4 

Total  for  3  strata  

226.0 

183.8 

146.7 

147  3 

*Field  of  Bloomington  Canning  Company. 


TABLE  8. — DISTRIBUTION  OF  NITRATE  NITROGEN  IN  THE 

FIELD  AT  BLOOMINGTON,  x  1921 
(Pounds  of  nitrate  nitrogen  per  acre-inch  of  soil) 


SOIL:    SWEET  CLOVER 


Layers 

Fallow 

Rye 

Oats 

Upper  sampling  stratum,  0  to  7  inches   

14.7 

4.9 

4.2 

Middle  sampling  stratum,  7  to  20  inches  

8.2 

7.2 

4.8 

4  6 

4.3 

4.6 

Substratum,  40  to  80  inches  

4.6 

aField  of  Bloomington  Canning  Company. 


two  corn  crops.  When  the  sweet-clover  crop  is  left  for  seed  production 
nitrification  is  advantageously  delayed,  with  the  result  that  fall  and 
spring  losses  are  reduced.  The  volume  of  organic  matter  is  not  so 
great  when  the  crop  is  left  for  seed  as  when  plowed  under  green  at  a 
somewhat  earlier  stage  of  maturity.  Under  many  circumstances,  how- 
ever, the  method  of  growing  the.  sweet  clover  to  maturity  is  less  advan- 
tageous than  plowing  it  under  green  in  the  spring  because  by  the  latter 
method  a  year  is  not  lost  for  growing  a  money  crop. 

The  magnitude  of  the  nitrogen  losses  that  may  occur  is  evident 
from  this  study  of  the  Bloomington  data.  They  are  perhaps  as  large 
here  as  any  that  would  be  met  with  under  almost  any  other  condition 
because  of  the  long  interval  occurring  between  plowing  the  green, 


302  BULLETIN  No.  285  [January, 

rapidly  decomposing  clover  crop  and  the  planting  of  the  succeeding 
grain  crop,  which  gives  time  for  the  drenching  rains  of  fall  and  spring 
to  be  effective  in  leaching  away  the  nitrates  formed. 

Thus  these  experiments  with  sweet  clover  at  Bloomington  add  a 
further  demonstration  of  the  efficiency  of  this  remarkable  plant  as  a 
producer  of  readily  available  nitrogen  for  soil  improvement. 


WINTER  LOSS  OF  NITRATE 

A  study  was  made  to  determine  the  amount  of  nitrate  lost  from 
the  40-inch  layer  of  the  soil  during  the  winter  and  early  spring  and 
the  effect  of  the  fertilizer  treatments  on  reducing  such  losses.  To  this 
end,  observations  were  made  on  certain  plots  of  a  number  of  the  soil 
experiment  fields  located  in  different  sections  of  Illinois. 

Plot  404,  which  receives  manure,  limestone,  and  phosphate,  the 
adjacent  check  (Plot  405),  and  the  nearest  sweet  clover  plot  (usually 
408)  receiving  crop  residues,  limestone,  and  phosphate,  were  selected 
on  eight  fields  in  northern  and  central  Illinois,  and  on  five  fields  in 
southern  Illinois. 

Fall  soil  samples  were  collected  in  November  and  December.  The 
Dixon,  Mount  Morris,  LaMoille,  and  Spring  Valley  samples  were  taken 
under  two  to  four  inches  of  snow.  The  remainder  of  the  samples  from 
the  northern  and  central  fields  were  taken  after  continuous  rains,  when 
the  soil  was  very  wet.  The  samples  from  the  southern  fields  were  taken 
after  heavy  rains. 

Table  9  records  the  pounds  of  nitrate  per  acre  in  the  fall  and 
spring  in  three  separate  sampling  strata,  namely,  the  upper  sampling 
stratum  (0  to  6%  inches),  the  middle  sampling  stratum  (6%  to  20 
inches) ,  and  the  lower  sampling  stratum  (20  to  40  inches) .  The  corre- 
sponding totals  are  found  in  the  last  three  columns  at  the  right.  The 
differences  between  fall  and  spring  results  represent  the  amounts  lost. 
Where  the  difference  resulted  in  a  gain  instead  of  a  loss  a  plus  sign 
appears  before  the  figures.  Manure  had  been  spread  on  Plot  404  on 
all  the  northern  fields,  which  presumably  would  increase  the  losses  for 
those  plots. 

It  should  be  understood  that  wide  variations  in  the  soil  types 
occur  among  these  fields,  and  in  some  cases  they  occur  within  a  given 
field.  The  types  range  in  character  from  shifting  sand  to  heavy  clay. 
Since  this  work  was  carried  out,  a  detailed  soil  map  has  been  prepared 
for  each  of  the  Illinois  soil  experiment  fields  (see  Bulletin  No.  273). 
From  these  maps  is  derived  the  information  regarding  the  soil  types 
on  the  plots  concerned  in  the  present  study,  as  given  in  Tables  9  and 
10. 

According  to  the  figures  of  the  last  three  columns  of  Table  9, 
which  represent  the  total  nitrates  to  a  depth  of  40  inches,  the  sweet- 


303 


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Plot  408 

304 


BULLETIN  No.  285 


[January, 


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1927}  HANDLING  SWEET  CLOVER  FOR  NITRATE  PRODUCTION  305 

clover  plots  (No.  408)  lost  the  smallest  amount  of  nitrate  (or  else 
actually  gained)  on  the  Dixon,  Mount  Morris,  Spring  Valley,  Aledo, 
and  Carthage  fields.  The  manure  plots  (No.  404)  lost  the  smallest 
amount  on  two  fields  of  this  group,  Oquawka  and  Kewanee.  The  check 
plots  lost  the  largest  amounts  on  the  Dixon,  Kewanee,  and  Carthage 
fields.  On  the  LaMoille  field  the  check  plots  and  the  manure  plots  lost 
about  the  same  amount. 

Pooling  together  all  the  fields  of  the  northern  and  central  group, 
the  total  loss  of  nitrate  nitrogen  (disregarding  that  absorbed  by  the 
growing  crop)  was  206  pounds  per  acre  on  the  sweet-clover  plots,  227 
pounds  on  the  manure  plots,  and  325  pounds  on  the  check  plots. 

In  connection  with  reduction  of  losses,  the  fact  should  be  consid- 
ered that  the  sweet  clover  itself  contained  at  least  100  pounds  and 
probably  150  pounds  of  nitrogen,  some  of  which  had  been  transformed 
from  nitrate  that  otherwise  would  have  been  lost.  In  some  cases  where 
large  amounts  of  nitrate  were  present,  the  loss  is  to  be  attributed  to 
some  extent  to  the  contour  of  the  field,  as  on  the  LaMoille  field.  On 
the  Kewanee  field,  a  growth  of  foxtail  was  probably  a  factor  in  pre- 
venting much  greater  losses  than  actually  occurred.  These  studies 
were  made  during  a  season  of  excessive  rainfall.  In  September  and 
March  there  was  about  four  times  the  normal  rainfall  and  on  most  of 
the  fields  only  a  little  less  occurred  in  April  than  in  March. 

On  the  southern  fields  which,  under  most  conditions,  contain  much 
less  nitrate  than  the  central  and  northern  fields,  the  losses  were  small 
both  as  to  absolute  quantity  and  on  a  percentage  basis.  There  is  a 
possibility  that  early  spring  nitrification  reduced  some  of  the  apparent 
losses.  The  manure  plots  gained  nitrate  on  three  fields,  which  sug- 
gests that  early  spring  nitrification  occurred,  and  where  no  crop  was 
present  to  absorb  the  nitrate,  it  accumulated.  On  the  sweet-clover  plots 
the  decrease  in  nitrate  might  be  misconstrued  as  representing  an  utter 
loss,  while  as  a  matter  of  fact  much  of  the  nitrate  was  immediately 
absorbed  by  the  growing  plants.  As  observed  in  the  other  studies, 
when  the  large  sweet-clover  crops  on  these  fields  are  taken  into  consid- 
eration, the  apparent  nitrogen  losses  become  actual  gains  of  about  100 
pounds  per  acre. 

Totaling  the  results  for  the  six  fields  of  the  southern  group,  the 
manure  plots  gained  collectively  27.5  pounds  per  acre  instead  of  losing 
61.9  pounds,  as  the  checks  did.  It  must  be  recognized  that  nitrate 
production  may  have  taken  place  thru  nitrification  of  the  manure, 
which  of  course  would  partially,  or  perhaps  wholly,  offset  the  nitrate 
loss. 

SUMMARY  AND  CONCLUSIONS 

In  order  to  obtain  the  maximum  fertilizing  value  of  sweet  clover, 
the  proper  handling  of  the  crop,  particularly  with  reference  to  the  time 
of  plowing  it  under,  must  be  given  consideration.  Observations  were 


306  BULLETIN  No.  285  [January, 

made  on  a  number  of  soil  experiment  fields  in  Illinois  of  the  develop- 
ment of  nitrate  in  the  soil  thru  the  decomposition  of  sweet  clover 
plowed  under  at  different  times  of  year. 

It  was  found  that  nitrification  of  both  fall-  and  spring-plowed 
sweet  clover  proceeded  rapidly  and  to  such  an  extent  on  one  field 
(Joliet)  as  to  furnish  nitrate  in  excess  of  the  requirements  of  a  large 
corn  crop.  The  spring  plowing  resulted  in  a  better  physical  condition 
of  the  soil  than  fall  plowing,  and  less  labor  was  required  in  preparing 
the  land  for  the  crop.  More  active  organic  matter  was  plowed  under 
on  the  spring-plowed  land,  and  this  is  one  of  the  most  important  con- 
siderations, especially  in  the  initial  use  of  sweet  clover.  Fall  plowing 
of  sweet  clover  is  frequently  desirable,  but  until  more  information  is 
available  as  to  methods  for  thoroly  killing  the  crop,  spring  plowing 
should  be  the  general  practice. 

Comparing  early  and  late  dates  for  spring  plowing  in  preparation 
for  corn,  it  was  found  that  early  plowing  gave  high  nitrate  at 
an  earlier  date  than  later  plowing.  At  both  the  Hartsburg  and  the  To- 
ledo fields  all  dates  of  plowing  permitted  a  rapid  nitrification  and  an 
accumulation  of  nitrate  sufficient  to  meet  the  needs  of  much  larger 
crops  than  were  produced. 

The  date  of  plowing  sweet  clover  in  the  spring  should  be  decided 
according  to  the  urgency  of  the  need  of  the  soil  for  active  organic  mat- 
ter, since  the  rapidity  with  which  sweet  clover  decomposes  after  spring 
plowing  insures  prompt  nitrification.  Light,  sandy,  and  open-textured 
soils,  and  those  deficient  in  organic  matter  should  have  the  sweet  clover 
plowed  as  late  as  consistent  with  good  soil  preparation  for  the  corn 
crop.  Heavy  soils,  such  as  clays,  loams,  and  in  general  those  soils  that 
have  grown  one  or  more  crops  of  sweet  clover,  are  not  likely  to  be 
materially  affected  for  corn  production  by  the  date  of  plowing  the 
sweet  clover. 

Summer-plowed  green  sweet  clover  nitrifies  rapidly  and  large 
amounts  of  nitrate  accumulate,  as  indicated  by  the  results  reported 
from  the  study  at  Bloomington.  Large  losses  of  nitrogen  result  if  no 
protective  crop  is  seeded.  Oats  and  rye  proved  efficient  in  converting 
much  nitrate  into  organic  nitrogen.  The  oats  were  more  valuable  in 
reducing  losses  than  the  rye,  owing  to  their  greater  fall  growth,  and 
to  the  fact  that  they  were  incorporated  into  the  soil  in  a  dry  condition 
rather  than  green. 

From  the  standpoint  of  nitrate  production,  this  manner  of  hand- 
ling sweet  clover,  whereby  it  is  allowed  to  grow  thru  the  second  season, 
is  of  no  particular  advantage  except  in  special  cases,  because  when 
plowed  down  in  the  spring  as  a  green  manure,  it  furnishes  sufficient 
soil  enrichment  without  sacrificing  a  year  to  the  growing  of  this  crop. 

Under  certain  conditions  nitrates  appear  to  concentrate  in  the  sur- 
face soil  by  rising  from  lower  layers.  That  such  a  rise  occurs  is  indi- 


1927]  HANDLING  SWEET  CLOVER  FOR  NITRATE  PRODUCTION  307 

cated  by  the  observation  that,  with  the  moisture  content  of  the  surface 
soil  remaining  below  the  point  for  permitting  nitrification,  the  nitrate 
content  actually  increased. 

In  not  a  few  cases  over  100  pounds  of  nitrate  nitrogen  was 
found  in  the  surface  soil,  even  in  the  presence  of  a  50-  to  65-bushel 
corn  crop  at  its  period  of  heavy  nitrate  absorption.  Such  amounts 
of  nitrate  nitrogen  should  be  converted  by  crops,  or  by  bacteria, 
to  some  fixed  form,  in  order  that  they  may  be  conserved  for 
use  by  succeeding  crops.  Under  farm  conditions,  weeds  in  the  corn, 
volunteer  grains,  and  any  crop  growth  on  the  land  in  late  summer  and 
fall,  serve  to  convert  much  nitrate.  If  a  legume  is  used  for  this  pur- 
pose rather  than  a  non-legume,  the  nitrogen  so  saved  is  more  rapidly 
and  completely  nitrified  the  following  year. 

Studies  made  on  thirteen  Illinois  experiment  fields  during  a  season 
of  unusually  heavy  rainfall  demonstrated  the  effectiveness  of  sweet 
clover  in  conserving  soil  nitrogen.  Less  nitrate  was  lost  on  sweet- 
clover  plots  than  on  corresponding  manure  plots  or  on  check  plots. 
These  results  were  obtained  where  sweet  clover  is  grown  as  a  green 
manure  in  a  four-year  rotation. 

Thus  these  investigations  show  that  the  method  of  handling  sweet 
4clover  plays  an  important  role  in  the  realization  of  the  full  fertil- 
ity value  of  the  crop. 


UNIVERSITY  OF  ILLINOIS-URBANA 


